In eukaryotic cells, the endoplasmic reticulum (ER) is the point of entry into the secretory pathway. After membrane translocation, properly folded and modified secretory proteins are packaged into transport vesicles and delivered to the Golgi complex. The molecular mechanisms involved in the selection and export of soluble secretory proteins into ER-derived vesicles for transport to the Golgi are not well understood. Currently, two differing models, "bulk flow" and "receptor-mediated" export have been proposed to describe how soluble secretory cargo exits the ER. The latter model requires specific interactions between soluble cargo and transmembrane receptors for export from the ER. The goal of this proposal is to test the hypothesis that a recently described ER-vesicle protein, Erv29p, acts as a transport receptor for the soluble secretory protein glyco-pro-alpha-factor (gpalphaf). A combination of biochemical, genetic, and biophysical approaches will be undertaken to dissect the role of Erv29p in gpaf transport. The proposed studies will attempt to identify export signals in gpaf that are sufficient for its transport from the ER. In addition critical regions in Erv29p: that are responsible for its interaction with soluble cargo will be identified. In vitro binding assays will be developed to recapitulate Erv29p-gpalphaf interactions in order to investigate potential regulatory mechanisms. Identification of the fundamental mechanisms that underlie ER to Golgi export may contribute to the treatment and prevention of certain human diseases.